Sharp Typical Distance and Exponential Small-Ball Bounds in One-Step-Cliff Nielsen Geometry

Abstract

Let $D=2^n$ and equip $\mathrm{PU}(D)$ with the one-step-cliff Nielsen metric, with quadratic metric coefficients one in Pauli directions of weights one and two and $D^2$ in all higher weights. We prove that the distance from the identity of a Haar-random element, normalized by $D$, converges to $\pi /\sqrt{3}$ in probability and in $L^p$ for every $1\le p<\infty$. Quantitatively, it lies within $O(D^{-1/8}(\log D{)}^{1/2})$ of this limit outside a set of Haar measure at most $\exp \{-\Omega (D^{7/4}\log D)\}$. For each fixed $0<x<\pi /\sqrt{3}$, the ball of radius $xD$ has Haar measure $\exp \{-{\Theta }_x(D^2)\}$; for $x>\pi /\sqrt{3}$, its complement has measure at most $e^{-c_x{D}^2}$ for some $c_x>0$. As $x\uparrow \pi /\sqrt{3}$, the lower and upper logarithmic rates are both asymptotic to $({\pi }^2/3-x^2{)}^2/(16\zeta (3))$. The small-ball upper bound follows from a comparison of Jacobi determinants, obtained by rescaling the linearized geodesic equations and applying Kato transport. Weyl integration reduces the remaining integral to an Abel-regularized logarithmic-energy estimate on the circle. A centered principal logarithm and concentration of the circular unitary ensemble eigenangle second moment give the distance upper bound.

38 pages, no figures